1 Technical Field
The present invention relates to rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2 Description of Related Art
There are known IPM (Interior Permanent Magnet) motors that have permanent magnets embedded in a rotor core. The IPM motors can use both reluctance torque and magnetic torque, thereby achieving high efficiency. Therefore, the IPM motors are particularly suitable for use in hybrid and electric vehicles.
An IPM motor generally includes a rotor and a stator. The rotor has a plurality of magnetic poles that are formed by the permanent magnets embedded in the rotor core. The stator includes an annular stator core and a multi-phase stator coil. The stator core is disposed in radial opposition to the rotor. The stator core has a plurality of stator teeth and a plurality of slots. The stator teeth each radially extend and are spaced from one another in a circumferential direction of the stator core. Each of the slots is formed between one circumferentially-facing pair of side surfaces of the stator teeth. The stator coil is comprised of a plurality of phase windings that are mounted on the stator core so as to be received in the slots of the stator core.
Moreover, there are disclosed, for example in Japanese Utility Model Application Publication No. JPH0629353Y2 and Japanese Patent Application Publication No. JP2010166810A (to be respectively referred to as Patent Documents 1 and 2 hereinafter), methods of reducing torque ripple in the IPM motors. More specifically, according to those methods, the stator core is formed by laminating in the axial direction a plurality of types of steel sheets having different circumferential lengths of distal end surfaces of the stator teeth. Consequently, it is possible to realize a skewed structure of the stator core, thereby reducing torque ripple.
However, in the case of applying the method disclosed in Patent Document 1, it is necessary to use a plurality of types of steel sheets having different stator tooth shapes to form the stator core. Moreover, the number of types of the steel sheets increases with the degree of skewing the stator core. Consequently, the man-hours required for processing the steel sheets is increased, resulting in an increase in the manufacturing cost of the stator core.
In the case of applying the method disclosed in Patent Document 2, the areas of the distal end surfaces of the stator teeth facing the rotor are reduced. Consequently, the amount of magnetic flux flowing through the stator teeth is accordingly reduced, thereby lowering the torque generated by the IPM motor.